SymptomsThe Panel is down and is showing "500 - Internal server error."
CauseSome of the files in the %plesk_dir%\admin\plib and %plesk_dir%\admin\htdocs folders are of different versions or do not exist.
ResolutionThese folders are to be compared with the ones on the test labs or working sets of the same version/build. Files are to be restored or replaced.
Note: If this solution did not help you, make sure you check out the main article: #114882:[HUB] 500 - Internal Server Error